This invention relates generally to supporting and transporting equipment and personnel on an elevated structure and more particularly to a lightweight mobile boom vehicle that operates in a narrow space such as along a bridge sidewalk.
It is often necessary to transport or support equipment and people over the edge of an elevated structure such as a bridge, oil rig, or ship. For example, it is often necessary to paint, inspect, hang conduit or work on structural components on the side or underneath a bridge. To support the equipment and workmen that must to be transported over the bridge, a basket is typically attached to the end of a crane or boom, or hung from ropes. The basket is placed on top of the bridge where it is loaded with the equipment and workman and then lowered over the side of the bridge to the location where the work must be performed. After the work has been completed in the present location, the basket is raised and the basket supports (e.g., ropes) are positioned in a second location. The basket is then reloaded with the equipment and workmen and then re-lowered over the side of the bridge. This process is repeated at each location along the bridge where work is preformed.
It is time consuming to raise and lower the basket each time it is repositioned, especially if work must be performed on many different areas. For example, if the entire bridge must be painted or if cable must be spanned underneath the bridge, then the basket must be repositioned many times during the job. It often can take more time to raise, reposition, and lower the basket than to actually perform the work at the location where the basket is lowered. Because many personnel, including the workers in the basket, support personnel, and traffic controllers are needed when the bridge work is being performed, the cost of completing the job increases proportionally with the time required to complete the work. In addition, with each raising and lowering of the basket, the chances of a dangerous mishap increase.
To reduce the amount of time required to complete the jobs performed on the sides and underneath elevated structures, trucks with attached boom assemblies are used to transport and support baskets over the side of the structure. The truck is positioned at some location, for example, in the roadway on a bridge. The boom assembly on top of the truck then raises and lowers a basket over the side of the bridge. When work is completed at the present location, the basket and boom are raised and the truck is then moved further down the road to a new bridge location. Thus, the boom truck eliminates having to untie and retie ropes at each new location on the bridge.
While boom trucks can reduce the time required to complete a bridge project, they also require a large amount of space and, therefore, cause congestion on bridge roadways. For example, when in use, a boom truck blocks at least one lane of traffic on the bridge roadway, increasing traffic congestion, and the risk of traffic accidents. Boom trucks also require flagmen to redirect traffic, further increasing the cost of performing the project.
Boom trucks also have a limited range of locations where they can position a basket. This is because the boom truck is relatively wide and, therefore, must be positioned on the roadway far from the side of the bridge. The boom assembly on top of the boom truck is then limited in the range of angles at which it can be directed over the side and underneath the bridge. In addition, because the truck is positioned on the bridge roadway, long boom members are needed to extend over the side of the bridge. Long boom members increase the amount of torque exerted on the boom truck when supporting equipment and workmen. Therefore, the boom truck must be heavy enough to prevent being tipped over. A heavy boom truck, however, cannot be driven on bridges with limited load capacity. In addition, longer boom assemblies are more expensive and also increase overall truck weight.
Stability can be increased by attaching outriggers on the outside of the boom truck. Typically, outriggers are hydraulically controlled and extend out the lateral sides of the truck to provide a wider truck base. The end of each outrigger has a fiat plate that presses on top of the bridge structure, holding the boom truck firmly in one location. Outriggers, however, by extending laterally out the sides of the boom truck, expand the area required for the truck to operate. An expanded boom truck operating area further obstructs traffic and limits the number of locations the truck can access. In addition, the metal plates on the bottom of each outrigger must be lifted off the ground and then reseated on the ground each time the boom truck is moved to a new location. Therefore, additional setup time is required each time the boom truck is moved.
It is often not possible for a boom truck to move next to the edge of an elevated structure. For example, a bridge often has railroad tracks or a sidewalk that runs along the outside edge. If a sidewalk runs along the side of the bridge, there are usually handrails or fencing on either side of the sidewalk that are too closely spaced together for a boom truck to fit. Even if a handrail did not exist, the sidewalk is typically elevated from the rest of the bridge roadway. Since the boom truck would then sit in an unstable non-level position, it is not possible for two wheels of the boom truck to ride on the sidewalk and the other two wheels to ride on the roadway.
Accordingly, a need remains for a small lightweight boom that is easily to move and that supports heavy loads in a wide variety of positions over the side of a bridge or other elevated structure.